President Donald Trump has raised the specter of using a government power called eminent domain to assume control of private land along the U.S.-Mexico border and build the border wall on which he campaigned and over which the federal government, unable to find any consensus, has shut down.

He brought up that possibility on Friday. As of Monday, there was no official announcement on using such special powers but at press briefing at the White House last week, Trump elaborated on how he has the authority to use eminent domain. The president has also announced he will address the nation in a live, televised address on Tuesday night at 6 p.m. PT.

“Without eminent domain you wouldn’t have any highways, you wouldn’t have any schools, you wouldn’t have any roadways,” Trump told reporters on Friday.

“What we’re doing with eminent domain is, in many cases, we make a deal upfront, and we’ve already done that … And if we can’t make a deal, we take the land and we pay them through a court process which goes actually fairly quickly. And we’re generous. But we take the land. Otherwise you could never build anything. If you didn’t use eminent domain you wouldn’t have one highway in this country. You have to use eminent domain.”

Later, Trump was quoted as saying he could grant himself the power to declare a national emergency and use “the military version of eminent domain.”

So what is eminent domain and how does it work?

Here’s a brief explanation of this special government power that dates back centuries.

Can the government really take land like this?

Yes. The U.S. Supreme Court case of Kohl v. United States in 1875 upheld long-existing law in the U.S. Constitution that grants the federal government special power to buy private land and transform it for public use. The Fifth Amendment ensures that land owners be given just compensation.

While the U.S. government has been allowed for a long time to acquire land for public use like parks, post office buildings, railroads, and airports, among other public spaces, eminent domain dates to the early 1200s in England.

So how does eminent domain actually work?

First, planners with the federal government must identify pieces of land it will need to achieve its building needs.

Second, the government works with appraisers to calculate the value of the land.

Third, the property owner is made an offer and if it is accepted, it’s a straightforward sale of land.

Are these offers always accepted?

What do you think?

So what happens if a land owner rejects the government’s offer?

More than a decade ago, the federal government began taking private property to build a border fence in Texas’ Rio Grande Valley. The Texas Tribune and ProPublica visited one stree ...

The federal government may seize the land before reaching an agreement. When an owner rejects an offer, the government may sue in court, which could either result in a long legal battle or a settlement that’s not satisfactory to the land owner.

In a special video report, The Texas Tribune explains how many Texas land owners along the Mexico border were forced to sell land. Those who would afford a lawyer were able to get more money compared to those who took on the government alone.

Has eminent domain ever been previously used to build a barrier along the U.S.-Mexico border?

Yes. In the final years of President George W. Bush’s administration, land owners in Texas had their property seized through eminent domain to build a border fence along the border with Mexico.

In 2007, the U.S. Department of Homeland Security filed more than 360 eminent domain lawsuits against Americans who owned land along the Mexico border, from Texas to California, The Texas Tribune reported. Ten years later, some of those lawsuits are still going on.

Could Trump declare a national emergency and use ‘the military version of eminent domain’?

It’s unclear what Trump meant, but the U.S. Constitution grants the secretary of a military department the authority to seize land and build fortifications, coast defenses or military training camps during wartime or when war is imminent.